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JOURNAL OF CLIMATE
VOLUME 20
Human Impact on Direct and Diffuse Solar Radiation during the Industrial Era
MARIA M. KVALEVÅG
Department of Geosciences, University of Oslo, Oslo, Norway
GUNNAR MYHRE
Department of Geosciences, University of Oslo, and Center for International Climate and Environmental Research, Oslo, Norway
(Manuscript received 10 July 2006, in final form 7 February 2007)
ABSTRACT
In this study the direct and diffuse solar radiation changes are estimated, and they contribute to the
understanding of the observed global dimming and the more recent global brightening during the industrial
era. Using a multistream radiative transfer model, the authors calculate the impact of changes in ozone,
NO2, water vapor, CH4, CO2, direct and indirect aerosol effects, contrails, and aviation-induced cirrus on
solar irradiances at the surface. The results show that dimming is most pronounced in central Africa,
Southeast Asia, Europe, and northeast America. Human activity during the industrial era is calculated and
accounts for a decrease in direct solar radiation at the surface of up to 30 W m⫺2 (30%–40%) and an
increase in diffuse solar radiation of up to 20 W m⫺2. The physical processes that lead to the changes in
direct and diffuse solar radiation are found to be remarkably different and the authors explain which
mechanisms are responsible for the observed changes.
1. Introduction
A decline in solar radiation has been observed at the
surface (Liepert 2002; Stanhill and Cohen 2001). Called
global dimming, this decline is presumed to be a consequence of an increased amount of scattering and absorbing aerosols and gases in the atmosphere from human activity and is likely to be linked to the reduced
pan evaporation (Roderick and Farquhar 2002). Alpert
et al. (2005) pointed out that the larger the population
the stronger the decline in surface solar radiation. They
estimated that urban activities between 1964 and 1989
explained the relatively large reduction in larger cities,
estimated at a maximum of ⫺0.41 W m⫺2 per year,
compared to the much smaller reduction in rural areas.
On a global scale, the decline in surface solar radiation
based on ground-based measurements sites is estimated
to be 7 W m⫺2 for the period 1961 to 1990 (Liepert
2002). A decline of 19 W m⫺2 for the United States in
particular is observed in the same study.
The abovementioned studies are based on data from
land observations carried out from the mid-1950s until
Corresponding author address: Gunnar Myhre, Department of
Geosciences, Postbox 1022 Blindern, 0315 Oslo, Norway.
E-mail: gunnar.myhre@geo.uio.no
DOI: 10.1175/JCLI4277.1
© 2007 American Meteorological Society
JCLI4277
the beginning of the 1990s. However, subsequent measurements of surface solar radiation taken between
1992 and 2002, at surface sites spread throughout the
world, provide evidence of increasing insolation at the
surface (Wild et al. 2005) called global brightening. This
brightening in surface measurements is also supported
by satellite measurements (Pinker et al. 2005).
Both global dimming and global brightening are
functions of changes in the sum of direct and diffuse
(scattered light) surface solar radiation. Therefore, it is
important to quantify the changes in the direct and diffuse solar radiation throughout the industrial era up to
the present, as well as to better understand the mechanisms behind them.
One factor that might explain change in surface solar
radiation is the presence of clouds, due to their large
variability and the extent to which they are influenced
by anthropogenic aerosols (Kaufman et al. 2002; Ramanathan et al. 2001). Reduction in total cloud cover is
normally consistent with increasing surface radiation.
Qian et al. (2006), however, show the opposite and explain this by the appearance of haze caused by the pollutants that prevent solar radiation from completely
penetrating down to the surface. Anthropogenic aerosols can potentially evaporate and inhibit cloud formation, especially in the case of absorbing aerosols
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KVALEVÅG AND MYHRE
(Ackerman et al. 2000), for example, atmospheric
brown clouds (Ramanathan et al. 2005).
In addition to cloud cover, solar surface radiation is
also affected by gases, aerosols (directly and indirectly),
and contrails, all of which absorb or scatter (or both)
solar radiation. Thus changes in these components will
alter both diffuse and direct solar radiation. Most of the
mechanisms responsible for the global dimming largely
go toward masking global warming, although some may
also contribute to it. The aim of this paper is to estimate
direct and diffuse solar radiation changes and explain
the observed changes in the total surface solar radiation
over the industrial era.
2. Method
We use a multistream radiative transfer model to accurately calculate the solar irradiances at the surface for
direct and diffuse radiation. The model contains the
discrete ordinate radiative transfer (DISORT) algorithm (Myhre et al. 2002; Stamnes et al. 1988) adopted
with eight streams. It includes absorption by atmospheric gases, clouds, and Rayleigh scattering and calculates downward solar radiation at the surface for direct and diffuse components. A spectral resolution of
four bands is used in the radiative transfer model (Myhre
et al. 2002). The surface albedo is spectrally computed
with solar zenith angle dependence included for the
bands in the model (Myhre et al. 2003). Vegetation data
for calculation of the surface albedo are from Ramankutty and Foley (1999). The meteorological data on
temperature and cloud cover are from the European Centre for Medium-Range Weather Forecasts (ECMWF)
for the year 2000. Radiative transfer calculations are
performed for monthly means with 3-h time step (aerosol hygroscopicity for the direct aerosol effects is calculated every third hour). Calculations for the indirect
aerosol effects are performed on daily data with 3-h
time step. Annual means are calculated based on the
monthly mean data (daily data for the indirect aerosol
effect). The model resolution is T42 (approximately 3°
⫻ 3°) and with 40 vertical layers. Our results are given
as the changes in direct and diffuse solar radiation at
the surface due to changes in the components that play
a role in global dimming and brightening. It uses the
same cloud cover for preindustrial times and the
present; thus, interannual variation is not taken into
account. In the version of DISORT used in this study
the delta-M scaling is not included. A large partition of
the scattered solar radiation may be in the forward direction of the direct solar radiation. Aerosols and
clouds scatter solar radiation mostly in the forward direction. Therefore, part of the direct solar radiation
4875
that is scattered into diffuse radiation may be at the
surface difficult to distinguish from the direct solar radiation. Note that in the model we calculate the diffuse
radiation as solar light that is scattered once or more.
For some of the components, the results are also given
in terms of radiative forcing at the top of the atmosphere to more clearly indicate to what extent they
mask or enhance global warming.
3. Results
Results for diffuse and direct solar radiation from
changes in gases, aerosols (direct and indirect effects),
and contrails are first shown separately over the industrial era. We then compile the data to derive the change
in the total downward solar surface radiation. As a reference, Fig. 1 shows the distribution of direct and diffuse surface radiation with the current abundance of
gases, aerosols, and clouds. All radiative transfer results
in this paper are for downward solar irradiances, and
their changes in figures and tables are all shown as
annual mean values.
a. Ozone
Over the last few decades, ozone has decreased in the
stratosphere and during the industrial era increased in
the troposphere (Gauss et al. 2006; Ramaswamy et al.
2001). In this work, we use data on ozone change since
preindustrial times from a chemistry transport model
(Oslo CTM2), which is part of a model intercomparison
(Gauss et al. 2006). Figures 2a and 2b show the change
in direct and diffuse solar radiation at the surface
caused by changes in ozone concentrations. Since
ozone absorbs solar radiation, direct and diffuse solar
radiation will be changed with the opposite sign of the
ozone changes. We find that ozone reductions in the
stratosphere dominate over ozone increase in the troposphere, except in the Tropics. Loss of ozone in the
stratosphere increases both direct and diffuse solar radiation at the surface, most strongly over Antarctica.
The values are relatively weak for ozone changes, and
globally the change in total ozone cause a brightening
at the surface of 0.18 W m⫺2 (see Table 1).
b. NO2
NO2 is a precursor of ozone and therefore contributes indirectly to changes in solar radiation. However, it
also has a direct effect because it absorbs solar radiation itself. Thus an anthropogenic increase in NO2
(Richter et al. 2005) reduces direct and diffuse solar
radiation (see Figs. 2c,d). The impact of NO2 on solar
surface radiation is more regional than that of ozone
due to its shorter lifetime. The NO2 vertical-averaged
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FIG. 1. The (a) direct,
(b) diffuse, and (c) total
solar irradiance at the
surface with the current
abundance of all gases,
direct and indirect aerosol effect, and contrails
and cirrus.
column is high in urban areas in northern America,
Southeast Asia, and Europe. These areas have had a
NO2 increase of ⬃1016 molecules cm⫺2 since preindustrial times, with maximum values at present of 1–2 ⫻
1016 molecules cm⫺2. Based on the changes in NO2 during the industrial era, our simulation shows that the
strongest values for the direct and diffuse components
are ⫺0.3 W m⫺2, and it contributes to dimming of
around 0.04 W m⫺2 in global and annual mean. A global mean shortwave radiative forcing is calculated for
the first time here for NO2 and is estimated to be 0.04
W m⫺2 or slightly more than 10% of the radiative forcing from tropospheric ozone (Gauss et al. 2006; Ramaswamy et al. 2001).
temperatures and thus a climate feedback. The trend is
also expected to have occurred before 1988, but this is
somewhat more difficult to quantify. To investigate
how water vapor may have affected current levels of
solar radiation, we increased the column water vapor
homogeneously with the change representative for the
period from 1988 to 2003, according to Trenberth et al.
(2005). Figures 2e and 2f show that the largest reduction in direct and diffuse solar radiation is at low latitudes, which has both the largest amount of water vapor
and strong incoming solar radiation. The results (see
Table 1) show a global reduction in direct and diffuse
solar radiation at the surface of ⫺0.15 and ⫺0.14 W
m⫺2, respectively. The local changes can be larger than
0.5 W m⫺2.
c. Water vapor
In addition to water vapor being the most important
greenhouse gas, it also absorbs in the solar spectrum
and can contribute to global dimming. There has been
an observed increase in water vapor during the last 15
yr (1988–2003), which can be seen as a linear trend of
1.3 ⫾ 0.3% decade⫺1 (Trenberth et al. 2005). This trend
has not been homogeneous around the globe and is
very likely a result of the increase in global surface
d. CH4
Methane also absorbs solar radiation (Collins et al.
2006). We have performed calculations for methane
with an increase in the concentration since preindustrial
time from 0.7 to 1.745 ppmv (Ramaswamy et al. 2001).
In the calculations the reduced mixing ratio with altitude in the stratosphere is taken into account. Most of
the methane absorption in the solar region takes place
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FIG. 2. The direct and diffuse solar radiation changes at the surface during the industrial era for (a),(b) ozone; (c),(d) NO2; (e),(f)
water vapor; (g),(h) CH4; (i),(j) CO2; (k),(l) sulfate; (m),(n) organic carbon and black carbon fossil fuel; (o),(p) biomass burning; (q),(r)
indirect aerosol effect; and (s),(t) contrails and cirrus. Note the dissimilar and in some cases nonlinear scales.
in the stratosphere; therefore, the diffuse solar radiation is barely changed at the surface (0.01 W m⫺2). The
direct solar radiation is globally reduced by 0.08 W m⫺2
at the surface. The pattern of the change at the surface
for CH4 in Figs. 2g and 2h can be related to the presence of cloud cover and partly to absorption of water
vapor. Strongest reduction is present at approximately
30°N and 30°S with a maximum of ⫺0.30 W m⫺2 in the
Saharan Desert. Methane is not one of the major contributors to the surface radiation change even though it
is stronger than NO2 on a global scale.
e. CO2
Collins et al. (2006) indicate that CO2 is a slightly
weaker contributor to dimming than CH4. We have
performed model simulations with a constant concentration in the atmosphere of 278 and 365 ppmv for preindustrial times and the present, respectively. The surface reduction in direct and diffuse solar reduction
shows a similar pattern as CH4 due to clouds and partly
due to overlapping absorption with water vapor (see
Figs. 2i,j). A large fraction of the absorption of CO2 is
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TABLE 1. Table of annual global direct, diffuse, and total solar
radiation change at the surface (W m⫺2).
Direct
radiation
Diffuse
radiation
Total
Tropospheric ozone
Stratospheric ozone
Total ozone
⫺0.06
⫹0.13
⫹0.07
⫺0.10
⫹0.20
⫹0.10
⫺0.15
⫹0.33
⫹0.18
NO2
H2O
CH4
CO2
Total gases
⫺0.01
⫺0.15
⫺0.08
⫺0.06
⫺0.22
⫺0.02
⫺0.14
⫺0.01
⫺0.02
⫺0.09
⫺0.04
⫺0.29
⫺0.09
⫺0.08
⫺0.31
Sulfate
Organic carbon
Black carbon
Biomass burning
Total direct aerosol effect
⫺1.55
⫺0.23
⫺0.14
⫺0.85
⫺2.77
⫹1.16
⫹0.17
⫺0.12
⫹0.21
⫹1.42
⫺0.39
⫺0.06
⫺0.26
⫺0.64
⫺1.35
Indirect aerosol effect
⫺0.03
⫺0.65
⫺0.67
Contrail
Cirrus
Total contrails and cirrus
⫺0.11
⫺0.19
⫺0.30
⫹0.09
⫹0.15
⫹0.24
⫺0.02
⫺0.03
⫺0.05
Total change
⫺3.32
⫹0.92
⫺2.38
Component
also in the stratosphere, which explains the small magnitude of the change of diffuse solar radiation. However, the stratospheric absorption by CO2 is smaller
than for CH4 and thus the reduction in the diffuse radiation is larger for CO2. Globally the direct radiation is
reduced by 0.06 W m⫺2 and the diffuse part is decreased by 0.02 W m⫺2.
f. Direct aerosol effect
Anthropogenic activity has increased the aerosol
content in the atmosphere. In this respect the most important components are sulfate, black carbon (BC), and
organic carbon (OC) from both fossil fuel combustion
and biomass burning. Aerosol datasets for both present
and preindustrial times are based on simulations from
the Oslo CTM2 chemistry transport model, which has
been part of a global aerosol comparison study
(AEROCOM; http://nansen.ipsl.jussieu.fr/AEROCOM/)
(Textor et al. 2006). Emissions of aerosols and their
precursors used in the calculations are from Dentener
et al. (2006). Aerosol optical properties are described in
Myhre et al. (2007). The aerosol simulations are performed with background aerosols, such as mineral dust
and sea salt. In this study no anthropogenic dust is included.
Sulfate aerosols scatter solar radiation. Therefore,
anthropogenic sulfate aerosols reduce direct solar radiation but increase diffuse radiation (see Figs. 2k,l).
The magnitude of the changes is up to 20 W m⫺2, with
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somewhat larger changes for direct solar radiation than
for diffuse radiation. The sum of these is the total
change in solar radiation at the surface, which for scattering aerosol is similar to the radiative forcing. Organic carbon from fossil fuel is also mainly in the form
of scattering aerosols, whereas BC from fossil fuel is
mostly absorbing. Therefore, BC aerosols—like atmospheric gases and unlike scattering aerosols—reduce
both direct and diffuse solar radiation. The fossil fuel
and biofuel OC and BC can affect direct and diffuse
solar radiation locally up to 10 W m⫺2 (see Figs. 2m,n).
In accordance with other works, we find that BC has a
much larger impact on solar radiation at the surface
than at the top of the atmosphere. Particles from biomass burning affect solar radiation over particularly
large regions in both Africa and southern America (see
Figs. 2o,p). These particles are estimated to have reduced direct solar radiation by up to 20 W m⫺2 during
the industrial era. Aerosols from biomass burning are
partly absorbent, so in a clear sky they mainly scatter
solar radiation, but under cloudy conditions with a high
degree of scattered light, absorption is quite strong. Figure 2p shows that the solar diffuse radiation differs in
sign over the globe for the biomass burning aerosols.
Globally, the total direct aerosol effect reduces the
amount of direct radiation by 2.8 W m⫺2 at the surface,
while the amount of diffuse radiation is increased by 1.4
W m⫺2 (see Table 1). The radiative forcing of the total
direct aerosol effect is ⫺0.42 W m⫺2.
g. Indirect aerosol effect
An increase in aerosols leads to more numerous but
smaller cloud droplets that reflect more solar radiation,
also known as the cloud albedo effect. To calculate the
change in effective radius from anthropogenic aerosols
(those included in the direct aerosol section), we follow the approach in Quaas et al. (2006). Their relationship between the concentration of cloud droplets and
aerosols is based on Moderate Resolution Imaging
Spectroradiometer (MODIS) data. All hydrophilic
aerosols, including natural aerosols such as subsize sea
salt and secondary organic aerosols, are included in
this approach. The radiative forcing due to the cloud
albedo effect is slightly stronger in our simulations
(⫺0.66 W m⫺2) than in Quaas et al. (2006). This is likely
to arise from a higher spatial resolution (in particular
vertical resolution) since larger variability in cloud liquid water content will strengthen the radiative forcing.
Figures 2q and 2r clearly show how the change in
diffuse radiation is much stronger than the direct component, since when clouds are present the direct sunlight is already scattered to diffuse radiation. Southeast
Asia is most influenced by the indirect aerosol effect. In
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this region, there has been an increase in sulfate and
carbonaceous aerosols compared to preindustrial times,
which reduces the effective radius. Here, the diffuse
radiation has its maximum reduction close to 10 W m⫺2.
Areas in Europe and North America are also influenced by the reduction of effective radius with a decline
of approximately 5 W m⫺2 for diffuse solar radiation at
the surface. We have not included the currently very
uncertain second aerosol indirect effect (Albrecht 1989;
Kaufman et al. 2005). By increasing the cloud amount,
this effect would reduce the direct solar radiation and
enhance the diffuse solar radiation.
h. Contrails and aviation-induced cirrus
Aircraft activity causes condensation trails (contrails), and under certain circumstances line-shaped
contrails can evolve into cirrus clouds (Minnis et al.
1998; Schroder et al. 2000). We have used the same
contrail cover as in Myhre and Stordal (2001) updated
with the increase in air traffic used in Sausen et al.
(2005). Stordal et al. (2005) estimated a trend of about
1%–2% decade⫺1 increase in cirrus cloud cover in regions with high levels of air traffic, and we adopted
their total increase in cirrus cloud cover and related
that to contrail cover. Figures 2s and 2t show changes in
direct and diffuse radiation from a combination of contrail cover and aviation-induced cirrus. The impact on
solar radiation shown in the figure is largest in northern
America, Europe, and in the flight corridor between
these two regions. The maximum reduction in direct
solar radiation is in northeast America with a magnitude of ⫺23 W m⫺2. The diffuse radiation increases in
the same region by 19 W m⫺2. The global mean changes
in solar radiation at the surface caused by contrails and
cirrus is calculated to be ⫺0.30 and ⫹0.24 W m⫺2 for
the direct and diffuse radiation, respectively.
i. Solar surface radiation change
Figure 3 shows the surface solar radiation change
from ozone, NO2, and water vapor, as well as the total
direct aerosol effect, cloud albedo effect, and aviationinduced contrails and cirrus. Changes in atmospheric
gases have only a relatively small impact on the reduced
total solar surface radiation in comparison with the
other effects over land areas with the strongest dimming. The atmospheric gases play a significantly larger
role to the dimming over the ocean and even become
the dominating component in some regions. In the Arctic and Antarctica the gases cause an increase in the
downward solar surface radiation and in particular at
high southern latitude the gases have a dominating role
for the total change in downward solar radiation. The
results presented here show that the direct aerosol ef-
4879
fect is a major contributor to global dimming, with a
reduced solar surface radiation of 10 W m⫺2 in industrialized and biomass burning areas. Since the cloud
albedo effect, only to a very small extent, had an impact
on the direct solar radiation, the total solar surface radiation is similar to the change in the diffuse surface
radiation with the greatest changes in the industrialized
areas. Contrails and aviation-induced cirrus can also
contribute to global dimming and reach a maximum
reduction in the solar radiation over North America of
4 W m⫺2.
j. Total changes
Figures 4a and 4b show the total of all anthropogenic
impact on direct and diffuse surface solar radiation.
Regions in central Africa, Southeast Asia, Europe,
and northeast America are most influenced by an anthropogenic reduction in the downward surface solar
radiation. The direct solar radiation is reduced by up to
30 W m⫺2 and the diffuse radiation is increased up to
20 W m⫺2 in these areas. The reduction in direct solar
radiation reaches 30% over certain parts of the eastern
United States and 40% over certain parts of China. The
diffuse solar radiation is strengthened by 30% in the
eastern United States. In the figures of total changes in
solar radiation we have summed each of the individual
components. Several model sensitivity experiments
were performed to investigate nonlinearities between
the effects that cause global dimming. The calculations
included two or more components causing the dimming
in the same experiment (including experiments with all
gases and all aerosols). The results showed near linearity between the mechanisms with a few exceptions in
biomass regions. The total radiation at the surface has
only been changed within 1%–3% compared to the additive approach. This can be explained by the partially
differing regional occurrence but also the fact that the
spectral behavior among the mechanisms varies.
The global mean changes in direct and diffuse radiation are ⫺3.3 and ⫹0.9 W m⫺2, respectively. The large
change in North America is mostly due to contrails and
cirrus from aircraft traffic and the direct aerosol effect,
while surface solar radiation in southeast Asia is mostly
influenced by direct and indirect aerosol effects. Figure
4c is the sum of direct and diffuse radiation and reveals
the total change of solar radiation at the surface. The
global dimming is pronounced over the continents, and
the maximum reduction in the total solar surface radiation is around 10 W m⫺2. The reduction in total solar
surface radiation of around 10 W m⫺2 over the United
States and China is lower by a factor of 2 compared to
that observed in 1961–90 in the United States (Liepert
2002) and also somewhat smaller than that observed in
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FIG. 3. The total solar radiation change (sum of direct and diffuse) at the surface during the industrial era for
(a) ozone, NO2, water vapor, CH4, and CO2; (b) direct aerosol effect; (c) indirect aerosol effect; and (d) contrails
and cirrus. Note the nonlinear scale.
China in 1955–2000 (Qian et al. 2006). There are several explanations for the difference. Over the United
States a large part of the increase in emissions of aerosols and their precursors occurred during the period
1961–90, with a reduction after 1990 (Lefohn et al. 1999;
Novakov et al. 2003; Streets et al. 2006). In our simulations, changes over the whole industrial era are considered. Therefore larger changes in total surface solar
radiation would be expected from the model than in the
measurements. On the other hand, reductions since
1990 have occurred. In addition, Alpert et al. (2005)
found that the global dimming was strongly linked to
the population. In our model, simulations that are performed on a 3° ⫻ 3° horizontal resolution fail to represent the maximum values. Based on MODIS aerosol
optical depth (AOD) (see Fig. 5), we find that within
our model grid, the AOD and thus the global dimming
may be higher by a factor of at least 2 than the average
in certain regions. Thus the lack of sufficient resolution
to resolve small scales in the model to a large degree
explains the difference between the model and the
measured global dimming because most of the measurements are made in urban areas. For other mechanisms, such as stratospheric ozone and aviation-induced
contrails and cirrus, changes have occurred over the
period with surface radiation measurements and for
these only reduced stratospheric ozone can contribute
to the reversal of global dimming.
4. Conclusions
Global dimming is a phenomenon that to a large degree takes place over certain land areas and the surrounding oceanic regions. We have shown that the spatial variability is large in our model results and that
some of the causal mechanisms of global dimming have
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FIG. 4. Total contribution
from all components on (a) direct, (b) diffuse, and (c) total
solar radiation change at the
surface during the industrial
era. Note the nonlinear scale.
a spatial resolution that cannot be fully resolved in global models. Brightening has been observed over the last
decade (Wild et al. 2005). The brightening is found to
be largest in many high latitude measurement sites in
accordance with our model results, and it is likely that
stratospheric ozone is one of several contributors. This
study shows that many mechanisms contribute to global
dimming and brightening. Furthermore, the mechanisms that contribute to this have a larger and dissimilar
impact on direct and diffuse solar radiation at the surface due to different physical processes. The reduction
in the direct solar radiation is as large as 40% in the
most industrialized and populated regions during the
industrial era. This reduction in direct solar radiation
may even be larger in more limited regions, which our
model cannot resolve, with spatial inhomogeneities in
the AOD, a substantial contributor to this reduction.
In this study it is shown that many atmospheric constituents contribute to the global dimming. The uncertainties vary among these components (Hansen et al.
2005; Ramaswamy et al. 2001). In particular the indirect
aerosols effect from reduced cloud droplet size and
aviation-induced cirrus are uncertain. Also the magnitude of the radiative effect of the direct aerosol effect
FIG. 5. Aerosol optical depth means for five cities (Beijing,
Tokyo, New York, London, and Oslo) at various spatial resolutions from MODIS data.
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(Schulz et al. 2006) and contrails (Sausen et al. 2005)
differs between various estimates. We have neglected
cloud cover changes due to aerosols or as a result of
climate feedback since their sign, spatial pattern, and
magnitude are even more uncertain than the components included in this study.
Our results also underscore the main distinction between human-induced and natural dimming as we have
shown that dimming resulting from anthropogenic
emissions has a distinct land signature. On the other
hand, dimming from natural sources—largely volcanic
eruptions leading to injection of aerosols in the stratosphere—is spread more evenly over ocean and land
areas. After the volcanic eruption at Mt. Pinatubo in
1991, for example, optical depth increased over both
land and ocean because of the large enhancement of
stratospheric aerosols with potential impact on the vegetation CO2 uptake (Roderick et al. 2001). The restriction of anthropogenic dimming to land areas suggests
that it may have a substantial impact on vegetation and
agricultural production. While plants respond to both
direct and diffuse radiation, they are most sensitive to
changes in diffuse radiation because it affects a greater
surface area of the plant (Roderick et al. 2001; Stanhill
and Cohen 2001). Although our results show an increase in diffuse radiation over the course of the industrial era, the magnitude of the decrease in direct sunlight is greater. Thus the consequences of the total global dimming (both direct and diffuse) for vegetation
and agricultural production need to be further investigated (Stanhill and Cohen 2001).
We show that solar absorption in the atmosphere
from human activity has contributed to the dimming
and is thus also likely to contribute to global warming.
This solar absorption is mainly due to absorbing aerosols and gases, such as NO2, tropospheric ozone, water
vapor, CO2, and CH4. However, this absorption is unable to explain the observed global dimming in industrialized regions and indicates that scattering components have a major role in global dimming. Thus, it is
likely that human-influenced scattering by aerosols and
clouds has contributed to a substantial offsetting of the
warming from the greenhouse gases.
Acknowledgments. We thank Lynn P. Nygaard and
Frode Stordal for valuable improvements in earlier versions of the manuscript. We appreciate the constructive
and useful comments from two anonymous reviewers.
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